The present invention relates to pipe fittings, in particular pipe fittings for electrofusion of pipes to form a pipeline. The present invention also relates to a pipeline incorporating a pipe fitting. The present invention yet further relates to a method of producing a pipe fitting.
Plastics pipes, in particular pipes made of polyethylene, are widely known for use in forming pipelines for conveying fluids such as water or natural gas. Such pipelines are made by joining together a succession of individual pipes using pipe fittings. It is known in the art to use pipe fittings incorporating a heating coil which surrounds the abutting ends of two pipes to be joined together. When an electrical current is passed through the coil, the current causes plastics material in the pipe fitting to melt and to be fused to the plastics material of the two pipes by melt displacement, thereby forming a welded joint which seals the two pipe ends together. Such known electrofusion pipe fittings suffer from the technical disadvantage that they are limited to fluid pressures in the pipeline of a maximum of 25 bar. At higher pressures, the electrofused fitting fails, leading to leakage from or catastrophic failure of the pipeline. Such low pressure pipelines employ single-layer polyethylene pipes.
EP-A-0396273 discloses an electro-fusion pipe coupler for such low pressure pipelines which comprises an outer shell and an electric resistance heating coil secured therein by an intervening layer of injection moulded thermoplastics material.
DE-C-3932807 discloses a welding muff and a process for manufacturing it in which the welding muff comprises a thermoplastic, tubular muff body with a heating wire winding on its inner surface and a reinforcing layer connected to the muff body. The reinforcing layer is configured for ensuring that the welding pressure is absorbed during welding and the service life is prolonged.
CH-A-683026 discloses a welding muff for plastic pipes in which a reinforcing mesh of steel wire or a perforated steel tube is embedded in the welding muff.
U.S. Pat. No. 4,770,442 discloses an electroweld fitting or collar which reliably applies contact pressure in the welding zone by providing a reinforcement in the form of a winding or a tube arranged on the outer surface of the body of the fitting, with the thermal coefficient of expansion of the reinforcement being smaller than that of the body.
U.S. Pat. No. 5,364,130 discloses a heat welder with coupling for tubular plastic pipes in which as well as a unitarily formed coupling part, there is disclosed a pipe coupling made with an inner part in the form of a socket or sleeve and separate, engageable, shell-like outer parts. With such an arrangement, a desired, external force closure in the vicinity of the junction point of the pipes is brought about by engagement of two shell-like halves of reinforcement shells to form a force transfer bridge.
GB-A-2299047 discloses the manufacture of electrofusion fittings in which a body of thermoplastic material is moulded over a shape-retaining structure comprising an electrical heating element in a layer of thermoplastic polymeric sheet material.
GB-A-1121850 discloses a jointing method of pipe shaped plastics and jointing materials in which an electric heating wire is wound on one end portion of a plastic sheath of a plastic sheathed cable and then a plastic sleeve is placed on the outside surface of the electric heating wire in an overlapping manner. Thereafter, the heating wire is supplied with electric current while applying compressive pressure to the entire overlapped portion, so that the plastic sheath and the plastic sleeve are united into an integral body by heating.
EP-A-0693652 discloses an electric welding pipe joint in which two or more layers of an outer cylindrical thermoplastic resin member are formed around a resistance wire-wound inner cylindrical member by successive injection moulding steps. In order to reduce the injection moulding time, it is disclosed that inner and outer cylindrical members constituted by three layers are successively moulded, rather than a single thicker layer.
EP-A-0303909 discloses the manufacture of an electrofusion coupler in which a tubular plastics preform is wound with a resistance heating wire and the heating wire is fully embedded in the preform. Thereafter, an outer encapsulating jacket is injection moulded around the preform.
The specifications referred to hereinabove suffer from the problem that they are not particularly directed to the manufacture of electrofusion pipe fittings which can be employed with high fluid pressures in the pipeline, typically greater than about 25 bar.
High pressure pipelines are also known in the art in which reinforced pipes are employed at fluid pressures above 25 bar. Such reinforced pipes have multilayer constructions, for example of multiple plastics layers which may additionally include a reinforcing layer including a metal. For such high pressure pipelines, it is known to use a mechanical coupling to join abutting ends of two adjacent pipes together in a sealed manner. Such mechanical couplings comprise an annular metal member which is bolted in a sealed manner around the opposed abutting ends of the pipes or, as shown in FIG. 1, is threadably connected to the pipe ends. As shown in FIG. 1, two pipes 2,4 having respective abutting ends 6,8 to be sealably joined together are surrounded by an annular metal coupling 10 having a threaded inner annular surface 12 into which the ends 6,8 are screwed. Such mechanical couplings have the technical problem that they are subject to corrosion of the metal, which over time can lead to failure of the coupling.
There is a need in the art for a pipe fitting which enables electrofusion of pipes together in a high pressure pipeline.
Accordingly, the present invention provides a pipe fitting for coupling together opposed ends of two plastics pipes by fusion welding, the pipe fitting comprising an annular body having an inner cylindrical surface of plastics material surrounding a cylindrical cavity for receiving opposed ends of two plastics pipes to be coupled together, an electrically conductive coil provided in said inner cylindrical surface and surrounding the cylindrical cavity, the coil having opposed ends, a pair of terminals each connected to a respective end of the coil and provided on the body for connection to a source of controlled electrical power, and an annular reinforcing member disposed in the body and surrounding the coil, the annular reinforcing member having a grooved surface which interfaces the annular body, the grooved surface defining a plurality of axially facing surfaces.
The present invention yet further provides a pipeline including at least two polyethylene pipes having opposed ends which are coupled together by at least one pipe fitting according to the invention, the inner cylindrical surface of the annular body being fusion welded to an outer cylindrical surface of each pipe end, wherein the annular body is composed of polyethylene and the annular reinforcing member has at least the same tensile strength in a hoop direction as that of the polyethylene pipes.
The present invention yet further provides a method of producing a pipe fitting for coupling together opposed ends of two plastics pipes by fusion welding, the method comprising the steps of: (a) forming an electroconductive coil around a substantially cylindrical core body; (b) moulding a first annular body part of the pipe fitting around the coil and the core body and disposing an annular reinforcing member around the first annular body part; (c) moulding a second annular body part of the pipe fitting around the reinforcing member and the first annular body part and the core body to form the pipe fitting; and (d) removing the core body from the pipe fitting.
Steps (b) and (c) may be carried out as a sequence of steps or as a single step.